The invention relates to the selection and switching of digital signals on a digital network.
This invention is applicable in any area in which it is necessary to switch asynchronous digital signals in which each digital signal is made up of a series of independent units of data, such as a series of frames, which may consist of information bits plus overhead bits, such as synchronizing preambles, control codes and the like.
The invention is especially useful in audio-visual production or transmission centers, in which switching networks are used to establish lines of communication between different equipment through link switching. In the following description, this application will be referred to as an example.
According to standards set by the C.C.I.R., the digital audio signals are exchanged between equipment in an audio-visual transmission or production by means of a serial interface, known as a UER/AES studio interface. The digital signal emitted by an interface is organized in frames of 64 bits, which allow the transmission of digitized left and right signals for a stereophonic program (24 possible information bits and 8 overhead bits per each of 2 channels). For example, with a sample signal frequency of 48 kilohertz, the line output is 3.072 megabits per second.
The signal sent on the line is coded in a biphase code, which provides important redundancy for the transmitted signal. This signal contains all the synchronization data necessary and, in particular, the timing bits and sampled signals. Synchronization at the level of the sampled signal is obtained by the use of preambles which break the coding principles of the biphase code.
The serial structure of the UER/AES interface lends itself well to the setting up of switching networks of the spatial type, which are a functional replica of the switching networks used for analog signals. The switching consists of sending the signal present on the appropriate network input to a given network output, with the assistance of a multiplexer, as shown in the drawing in FIG. 1.
This switching device consists mainly of a multiplexer (2) which has, for example, 16 inputs Y.sub.0, Y.sub.1, . . . , Y.sub.15, and one output. It also has 16 input stages 4.sub.0, 4.sub.1, . . . , 4.sub.15 to adapt a signal received from a standardized interface to make it compatible with the multiplexer, for example, to convert a signal which follows the RS432 standard, received from the UER/AES interface, into a TTL signal. The switching device has an output stage (6) in order to adapt the signal emitted by the multiplexer (2) to make it compatible with the standarized interfaces.
This spatial switching system has the advantage, by comparison with the temporal type of switching used in the standard way in automatic telephone switching, of not requiring exact synchronization between the digital signals to be switched.
The switching system shown in FIG. 1 thus makes it possible to provide switching between the digital signals emitted from interfaces, whose frames are not in phase, either because the sample frequencies are slightly different, or because the transit time between the equipment and the cables used do not match.
Generally, switching between two digital audio signals is accomplished in a crude fashion, without filtering. This can give rise to sound discontinuities in the form of a click. These defects, whose maximum amplitude varies with the level of the sound program, are acceptable in a number of significant applications. However, it may happen that this switching leads to a high-amplitude click, even in the presence of a low-level sound signal.
These defects may appear during switching between two asynchronous digital audio signals. Such a situation is shown in FIG. 2. A switching is carried out between a signal, S1, made up of a series of frames i, i+1, i+2, . . . and a signal, S2, made up of a series of frames j, j+1, j+2, . . . The switching takes place during the i+1 frame for the S1 signal and the j+1 frame for the S2 signal.
The SQ sequence between the i frame and the j+2 frame of the resulting signal, SR, is an abnormal sequence, first of all, due to its length, and secondly, because it is not a frame. In a conventional arrangement, an appropriately designed receiver can recognize this anomaly, due to the discontinuity of the frame timing in the resulting signal, SR, and use known methods of simulation, for example, by repetition or interpolation, to re-create the missing sample signals in the resultant signal, SR.
In contrast, when the two signals switched are synchronous, no timing discontinuity can be detected by the receiver. FIG. 3 illustrates such a switching.
Digital signals S1 and S2 are synchronous. The SR signal which results from the switching is therefore made up of a series of frames, because the SQ sequence, composed of the beginning of the i+1 frame of the S1 signal and the end of the j+1 frame of the S2 signal, also has a frame structure.
The receiver which receives the SR resultant signal cannot therefore detect the switching and interprets the erroneous data contained in the abnormal SQ sequence. The switching can be then indicated, during the sound reconstruction of the digital signal, by a loud high amplitude click.
The results of the switching during the transmission of sample data from a frame is shown on FIG. 4, in the case where the frames of the two digital audio signals are synchronous. The data elements, D1 and D2, are contained in the frames i+1 and j+1, respectively, (cf.: FIG. 3) and each represents the coded value of the sample of an audio signal. These sample signals are additionally coded in pairs, typically on 16 bits, that is, within the interval -32768 to +32767.
The data element D1, with a value of 0, represents a sample of the 0 level, emitted in complete silence. Similarly, data element D2, made up of a series of "1" symbols, represents a sample of the -1 level, that is, a negative audio signal with a very low relative amplitude.
The data element, DR, resulting from the switching between D1 and D2 is a sample signal with a relatively high amplitude, that is, a sample signal for a level very different from each of the two signals which have been switched. During the reconstruction of the audio signal, a high amplitude parasite sample signal, which is perceived as a loud click, therefore appears upon switching.
A method for reducing this problem may consist of carrying out the switching at a predetermined point in the frames, for example, during a preamble of auxiliary data or during less significant bits of sample signals. However, such a method, comparable to the one used in video applications in which switching is carried out while suppressing the raster, would necessitate the extraction of the frame timing of at least one of the two signals. The latter would singularly complicate the switching method and would not make it possible to improve the situation when the frames are not synchronous.